Pharmacogenetic roles of CYP2C19 and CYP2B6 in the metabolism of R- and S-mephobarbital in humans.

OBJECTIVES AND METHODS: We assessed the relationship between the metabolism of R- and S-mephobarbital (MPB) and genetic polymorphisms of cytochrome P450 (CYP) 2C19 and CYP2B6. Nine homozygous extensive metabolizers (homo-EMs, 2C19*1/2C19*1) of CYP2C19, ten heterozygous EMs (hetero-EMs, 2C19*1/2C19*2, 2C19*1/2C19*3) and eleven poor metabolizers (PMs, 2C19*2/2C19*2, 2C19*3/2C19*3, 2C19*2/2C19*3) recruited from a Japanese population, received an oral 200 mg-dose of racemic MPB. Blood and urine samples were collected, and R-MPB, S-MPB and the metabolites, phenobarbital (PB) and 4'-hydroxy-MPB, were measured. Each subject was also genotyped for CYP2B6 gene. RESULTS: The mean area under the plasma concentration-time curve (AUC) of R-MPB was 92-fold greater in PMs than in homo-EMs. R/S ratios for AUC of MPB were much higher in PMs than in EMs (homo- and hetero-). The cumulative urinary excretion of 4'-hydroxy-MPB up to 24 h postdose was 21-fold less in PMs than in homo-EMs. The metabolic ratio of AUCPB/(AUCS-MPB + AUCR-MPB) was higher in PMs than in EMs (homo- and hetero-). In addition, this metabolic ratio was lower in the carriers of CYP2B6*6 compared with that in its non-carriers. CONCLUSIONS: Our results indicate that the 4'-hydroxylation of R-MPB is mediated via CYP2C19 and that the rapid 4'-hydroxylation of R-MPB results in a marked difference in the pharmacokinetic profiles between R-MPB and S-MPB in the different CYP2C19 genotypic individuals. In addition, a minor fraction of the interindividual variability in PB formation from MPB may be explainable by the CYP2B6*6 allele.

Prediction of adsorption from multicomponent solutions by activated carbon using single-solute parameters. Part II--Proposed equation.

Prediction of multicomponent adsorption is still one of the most challenging problems in the adsorption field. Many models have been proposed and employed to obtain multicomponent isotherms from single-component equilibrium data. However, most of these models were based on either unrealistic assumptions or on empirical equations with no apparent definition. The purpose of this investigation was to develop a multicomponent adsorption model based on a thermodynamically consistent equation, and to validate that model using experimental data. Three barbiturates--phenobarbital, mephobarbital, and primidone--were combined to form a ternary system. The adsorption of these barbiturates from simulated intestinal fluid (without pancreatin) by activated carbon was studied using the rotating bottle method. The concentrations, both before and after the attainment of equilibrium, were determined with a high-performance liquid chromatography system employing a reversed-phase column. The proposed equation and the competitive Langmuir-like equation were both fit to the data. A very good correlation was obtained between the experimental data and the calculated data using the proposed equation. The results obtained from the original competitive Langmuir-like model were less satisfactory. These results suggest that the proposed equation can successfully predict the trisolute isotherms of the barbituric acid derivatives employed in this study.

Role of CYP2C19 in stereoselective hydroxylation of mephobarbital by human liver microsomes.

The 4-hydroxylation of mephobarbital enantiomers was investigated by using human liver microsomes from the extensive metabolizers (EM) and poor metabolizers of CYP2C19. The 4-hydroxylase activity of R-mephobarbital in the EM microsomes was >10 times higher than that of S-mephobarbital. In the poor metabolizer microsomes, the 4-hydroxylase activity of R-mephobarbital was much lower than that in the EM microsomes, and the ratio of 4-hydroxylase activity of R-mephobarbital to that of S-mephobarbital was also lower than that in the EM microsomes. Moreover, the 4-hydroxylase activity of R-mephobarbital showed a high correlation (r = 0.985, p<0.001) with the 4'-hydroxylase activity of S-mephenytoin in a panel of nine human liver microsomes. Anti-CYP2C antibody inhibited R-mephobarbital 4-hydroxylase activity by 85% of the control activity. R-Mephobarbital competitively inhibited S-mephenytoin 4'-hydroxylase activity (K(i) = 34 microM), while S-mephenytoin inhibited R-mephobarbital 4-hydroxylase activity (K(i) = 103 microM). Among the seven cDNA-expressed CYPs studied, only CYP2C19 catalyzed R-mephobarbital 4-hydroxylation. These findings suggest that the 4-hydroxylation of mephobarbital catalyzed by CYP2C19 is preferential for R-enantiomer in human liver microsomes.

Diminished mephobarbital anticonvulsant action following diphenhydramine pretreatment.

Diphenhydramine and other antihistamines produce biphasic effects on drug disposition and lower seizure threshold, thereby potentially diminishing the efficacy of anticonvulsants such as mephobarbital. Accordingly, the influence of diphenhydramine (50 mg/kg, IP) pretreatment on the anticonvulsant activity of mephobarbital (50 mg/kg, IP) was determined in adult female Swiss-Webster mice given pentylenetetrazol (SC). Diphenhydramine lowered the pentylenetetrazol convulsive dose (CD50) by 60%. Administration of diphenhydramine in combination with mephobarbital produced a 65% decrease in the CD50 of pentylenetetrazol in comparison with that of animals given mephobarbital plus pentylenetetrazol. Pharmacokinetic evaluation of mephobarbital blood level data indicates that the mechanism responsible for the observed interaction between diphenhydramine and mephobarbital involves a decrease in mephobarbital uptake from the administration site.

Formation of active metabolites of anticonvulsant drugs. A review of their pharmacokinetic and therapeutic significance.

All of the commonly used anticonvulsants drugs, except possibly primidone, are cleared from the human body mainly by metabolism. The metabolites of phenytoin, phenobarbital and ethosuximide have so far not been shown to possess significant pharmacological activity. However, carbamazepine-10,11-epoxide, derived from carbamazepine, has anticonvulsant activity comparable with that of its progenitor, while oxcarbazepine, a new anticonvulsant congener of carbamazepine, is essentially a prodrug for its 10-hydroxy derivative. Valproic acid forms numerous metabolites through a variety of pathways; 2-en valproic acid, a beta-oxidation derivative, probably contributes to its anticonvulsant action, though the extent of the contribution is uncertain. Another metabolite, 4-en-valproic acid, has been considered a possible hepatotoxin and teratogen. N-Methyl-phenobarbital and primidone, though both anticonvulsants in their own right, are metabolised to phenobarbital, which probably mediates much of their antiseizure effect. Primidone also yields the weaker anticonvulsant phenylethylmalonamide. The various benzodiazepine anticonvulsants form numerous metabolites, some of which possess both antiseizure and other forms of pharmacological activity. As yet, there is little understanding of how best to interpret simultaneous plasma concentration measurements of anticonvulsant drug and its active metabolite (or metabolites) in the clinical situation, and the possible roles of anticonvulsant metabolites in the idiosyncratic toxicity of these drugs remain largely unexplored.

Plasma antiepileptic drug concentrations during pregnancy.

Steady-state plasma antiepileptic drug (AED) concentrations were measured at intervals throughout pregnancy and during the postnatal period in 105 women who underwent 134 pregnancies. Phenytoin (PHT) dosage had to be increased in 85% of pregnancies in which the drug was received, carbamazepine (CBZ) dosage in 70%, and phenobarbital (PB) or methylphenobarbital (MPB) dosage in 85%, in an attempt to prevent or correct a fall in plasma concentrations of the respective drugs as pregnancy progressed. The altered disposition of the AEDs usually began in the first 10 weeks of pregnancy (often before epileptic pregnant women are referred for neurological supervision), and had returned to baseline value within 4 weeks of childbirth in two thirds of the women receiving PHT. The return to the nonpregnant situation appeared to be slower for CBZ, PB, and MPB. In women studied during more than one pregnancy, the changes in AED dosage to plasma concentration ratios tended to be greater in the first than in the subsequent pregnancies. Full seizure control prior to pregnancy was associated with a more favorable outcome for freedom from seizures during pregnancy. However, the plasma level monitoring-dosage adjustment policy produced no marked improvement in overall seizure control in pregnancy. This may have occurred because some patients were seen too late in their pregnancies for the policy to have been applied optimally.

The influence of age and gender on the stereoselective metabolism and pharmacokinetics of mephobarbital in humans.

In this clinical investigation, four groups of subjects (eight young women and eight young men [age range, 18 to 25 years], and eight elderly women and eight elderly men [greater than 60 years of age]) received single oral doses (400 mg) of racemic mephobarbital. The apparent total body clearance of R-mephobarbital was much greater and the elimination half-life was much shorter in the young men compared with the other three groups. This enantiomer displayed an age-dependent gender effect and a gender-dependent age effect in its metabolism. The apparent total body clearance of the S-enantiomer was much lower than that of the R-enantiomer in all subjects and did not differ between subject groups, although the elimination half-life was slightly but significantly shorter in young males. A consequence of these enantiomeric differences was an apparently enhanced stereoselectivity in the metabolism of mephobarbital in young men. These substantial influences of age and gender on the stereoselective disposition of mephobarbital are consistent with recent findings concerning the expression and regulation of cytochrome P450 enzymes.

Stereoselective metabolism and pharmacokinetics of racemic methylphenobarbital in humans.

The stereoselectivity of the metabolism and pharmacokinetics of methylphenobarbital (MPB) was studied in six healthy adult male volunteers given single oral doses of the racemic drug. All of the volunteers were phenotypically extensive metabolizers of the drug. The R- and S-enantiomers of MPB were analyzed in plasma by an enantioselective HPLC method, and the enantiomers of the 4-hydroxy-MPB metabolite in urine by a similar procedure. The (R)-MPB was extensively hydroxylated, with an average of 49.56% of that enantiomer being recovered in urine as (R)-4-hydroxy-MPB. Only 7.16% of the (S)-MPB was converted to the corresponding hydroxy metabolite. The extensive hydroxylation of (R)-MPB resulted in rapid elimination of this enantiomer, with a terminal plasma half-life of 7.52 +/- 1.70 (SD) hr. The (S)-MPB, the only recognized metabolites of which were (S)-4-hydroxy-MPB and phenobarbital (PB), was eliminated very slowly [t1/2, 69.78 +/- 14.77 (SD) hr]. The oral clearance of (R)-MPB (0.470 +/- 0.184 (SD) liters/hr/kg) was much higher than that of (S)-MPB [0.017 +/- 0.001 (SD) liters/hr/kg]. The extreme differences in metabolic fate and pharmacokinetics of the enantiomers of MPB are interesting. Most of the circulating PB seemed to be derived from (S)-MPB. In other respects the pharmacodynamic implications of these pharmacokinetic differences are unclear, because the relative anticonvulsant potencies of the enantiomers of MPB are unknown.